One of the most important parameters in oil exploration and production, which has defied measurement by conventional logging tools, is the fluid permeability of rock formations penetrated by the borehole. Although permeability can be estimated by measuring the formation porosity in combination with other geologic information, which can be related to permeability, and by measuring formation permeability from core samples or from intermittent-sampling formation testing tools, no dependable method has been found to directly log formation permeability with a continuous logging tool.
It has been proposed that the acoustic tube wave traveling in a borehole is responsive to the permeability of the surrounding formations. The tube wave, also commonly referred to as Stoneley wave, is a guided surface wave which travels in a direction axially of the borehole on the surface of the borehole wall. In an important work on acoustics and geophysics, Underground Sound, Elsevier (Amsterdam, 1983), J. E. White has explained that a low frequency tube wave in a permeable borehole can be viewed as a pulsating pressure which displaces fluid alternately into and out of the borehole wall. He also set forth, in chapter 5, some of the basic mathematical expressions which relate formation permeability to the acoustic tube wave velocity.
However, attempts to use tube waves to measure permeability in real boreholes have had only limited success. Personnel at Mobil Research and Development Corp. have opted not to use existing acoustic logging tools, but to use a specially designed long spacing acoustic logging tool to measure tube waves, as described in D. M. Williams et al., "The Long Spaced Acoustic Logging Tool" SPWLA 25th Annual Logging Symposium, June 10-13, 1984. They have collected well data confirming that there is a relationship between tube wave velocity and permeability. In the article of J. Zemanek et al., "New Developments in Acoustic Logging," Proceedings of the Indonesian Petroleum Association Fourteenth Annual Convention, October 1985, pp. 565-586, the measured tube wave amplitude and velocity in tested wells were found to be qualitatively related to measured formation permeability, but the measurements could not be made quantitatively correct. Likewise, it has been suggested in the literature that the tube wave amplitude should be a good measure of permeability. J. H. Rosenbaum, "Synthetic Microseismograms: Logging in Porous Formations," Geophysics, Vol. 39, No. 1 February 1974).
Others at Shell have reported correlations between the acoustic energy of that part of the measured wavetrain corresponding in velocity to the tube wave, as measured by a commercially available Schlumberger acoustic tool, with measured formation permeability. J. J. Staal and J. D. Robinson, "Permeability Profiles From Acoustic Logging," SPE Paper 6821, 52nd Annual Fall Technical Conference, Oct. 9-12, 1977.
U.S. Pat. No. 4,575,828 to Williams describes a method which purports to determine both the permeability due to the rock matrix and the permeability due to fractures in the rock, using the ratio of tube wave amplitudes and travel times, respectively, measured at two spaced apart receivers in the borehole. Although this method obtains results which may correlate with field data in some respects, it does not seem to account for effects of the rock matrix or fluid on the acoustic measurement.
Despite the strong interest in linking acoustic waves and permeability, as shown by the above, the reported experimental results and field measurements have not heretofor yielded a clear theoretic or quantitative connection which can reliably predict permeability in real oil wells.
Accordingly, it is an object of the present invention to provide an improved method for determining formation permeability using acoustic data obtained from a continuous logging tool. It is also a specific object of the invention to incorporate information of the formation matrix and fluid with measured tube wave data to determine permeability. Among other objects of the invention which will become apparent from the following description, it is an object to provide practical methods to modify logged waveform data to improve the sensitivity to permeability while reducing the influences of other borehole phenomena.